This project will examine the interaction between sex hormones, opiate receptors and endogenous opioid compounds in the medial preoptic area (MPOA) of the rat during perinatal development and during adult life. Opiate receptors in this region exhibit a sex difference shortly after birth and respond dynamically to hormonal changes during the estrous cycle of females. The development of MPOA opiate receptors may be sex hormone dependent. This project will examine the ontogeny of MPOA opiate receptors by quantitative densitometry of [3H]naloxone autoradiographs of the MPOA from intact male and female rats from the time of birth to 6 days of age (the period during which the sex difference develops), and in castrated or tamoxifen-treated males and testosterone- or dihydrotestosterone-treated females during the early postnatal period. The ontogeny of the endogenous opioid compounds, methionine- and leucine enkephalin and Beta-endorphin, will be examined during the early postnatal period using immunohistochemical procedures. Autoradiographic techniques will also be used to study the influence of various sex hormones which mimic the estrous cycle on MPOA opiate receptor binding in adult gonadectomized females and males. The adult hormonal manipulations will elucidate the hormonal mechanism involved in opiate receptor density variations during the estrous cycle and examine the "fixed" or dynamic nature of opiate receptors in the male hypothalamus. Endogenous opioid systems may be involved in the control of reproductive behavior and/or hormonal cyclicity. These investigations of the ontogeny and adult function of hypothalamic opioid systems represent an initial step in our understanding of the control mechanism. The results may elucidate the mechanism of normal and aberrant reproductive function and of narcotic effects on reproductive function.